Nowadays, certain motor vehicles are equipped with “hands free” access; that is to say the authorized user of the vehicle no longer needs a key to open the doors and other openable panels (hood, trunk, etc.) of his vehicle. In place of a key, he possesses an identification badge (or remote control) with which the vehicle electronic system interacts.
To invoke the opening of a door, for example, the driver approaches the handle of the door. A capacitive presence sensor, in this instance a charge-transfer capacitive sensor situated in the handle, detects the presence of the drivers hand. This sensor is connected to the electronic computer of the vehicle (ECU: English abbreviation for “Electronic Control Unit”) and sends it a presence detection signal. The electronic computer of the vehicle has beforehand identified the user as being authorized to access this vehicle, or alternatively, subsequent to the reception of this detection signal, it undertakes this identification. Accordingly, it sends by way of an LF (English abbreviation for “Low Frequency”) antenna an identification request to the badge (or to the remote control) worn or carried by the user. This badge sends in response, by RF (radio frequency) waves, its identification code to the electronic computer of the vehicle. If the electronic computer recognizes the identification code as that authorizing access to the vehicle, it triggers the opening of the door. If, on the other hand, the electronic computer has not received any identification code or if the identification code received is erroneous, opening does not occur.
As illustrated in FIG. 1, a capacitive sensor 3 such as this consists of an electrode 4 integrated into the handle 6 of the door and of a second electrode linked to ground. This second electrode can include a part of the body of a user and of a close environment linked directly or indirectly to ground. This may be, for example, the hand M of the user, the presence of which near the handle 6 of the door has to be detected.
When the user's hand M approaches the handle 6 of the door, that is to say it passes from position 1 to position 2 along the direction of the arrow illustrated in FIG. 1, the capacitance CX of the electrode 4 integrated into the handle 6 increases. The variation ΔCX is measured with the aid of a reference capacitance CS, situated on a printed circuit 5 connected to the electrode 4. If the value of the capacitance CX crosses a threshold, this gives rise to the validation of the detection. Indeed this means that the users hand M is in position 2, on the handle 6 of the door or sufficiently close to this handle 6 and that he is requesting access to the vehicle.
From the prior art it is known that the charge-transfer capacitive sensor 3 makes it possible to measure the variation ΔCX of the capacitance CX of the electrode 4 integrated into the handle 6 of the door by performing a charge transfer consisting of a large number of charges and of discharges of this capacitance CX into the reference capacitance CS, until a fixed threshold of voltage is attained across the terminals of the reference capacitance CS. The estimation of the variation ΔCX of the capacitance CX of the electrode 4 with respect to the previous cycle is carried out on the basis of the variation of the number of discharges of the capacitance CX of the electrode 4 into the reference capacitance CS that had to be performed so as to attain this threshold of voltage across the terminals of the reference capacitance CS. These capacitive sensors 3 involve switching means which make it possible to direct the current so as firstly to charge the capacitance CX of the electrode 4 by way of the supply voltage and thereafter to discharge it into the reference capacitance CS. The charge transfer, that is to say the succession of charges and discharges, according to the prior art, and illustrated in FIG. 2, breaks down into four steps:                1st step: the first step consists in charging the capacitance CX of the electrode 4 on the basis of the supply voltage VCC. Accordingly the first switch S1 is closed and the second switch S2 is opened.        2nd step: once charging has finished, the first switch S1 is opened.        3rd step: then the discharging of the capacitance CX of the electrode 4 into the reference capacitance CS can begin. Accordingly, the first switch S1 remains open and the second switch S2 is closed.        4th step: once discharging has been carried out, the second switch S2 is opened.        
The charge transfer is repeated until the voltage VS across the terminals of the reference capacitance CS attains the threshold voltage VTH. The number of discharges x of the capacitance CX of the electrode 4 to the reference capacitance CS required to attain this threshold VTH gives an image of the capacitance CX of the electrode 4. The reference capacitance CS is thereafter completely discharged by way of the switch S in preparation for the next measurement.
A counter of the number of discharges x and a microcontroller (neither of which is represented in FIG. 2) make it possible to determine the capacitance CX of the electrode 4.
The equation governing the operation of the capacitive sensor 3 is the following:
            V      S        ⁡          (      x      )        =                              C          X                          C          S                    ×              V        CC              +                            V          S                ⁡                  (                      x            -            1                    )                    ×              (                  1          -                                    C              X                                      C              S                                      )            
The evolution of the voltage VS across the terminals of the reference capacitance CS constitutes a mathematical series according to the number of discharges x of the capacitance CX of the electrode 4 to the reference capacitance CS, and is given by equation (1):
                                          V            S                    ⁡                      (            x            )                          =                              V            CC                    ×                      (                          1              -                                                (                                      1                    -                                                                  C                        X                                                                    C                        S                                                                              )                                x                                      )                                              (        1        )            
At the end of the charge transfer, the voltage VS across the terminals of the reference capacitance CS has reached the threshold voltage VTH, and a number of discharges x is obtained, defined by equation (2):
                    x        =                              -                                          C                S                                            C                X                                              ×                      ln            ⁡                          (                              1                -                                                      V                    TH                                                        V                    CC                                                              )                                                          (        2        )            
Th is defined as a detection threshold, corresponding to a number of charge transfers between the two states of the capacitance CX of the electrode 4, that is to say between CX and CX+ΔCX.
Th is equal to the variation of the number of discharges x, between the value of the capacitance CX and the value of the capacitance CX+ΔCX.
Consequently:
  Th  =                    -                              C            S                                C            X                              ×              ln        ⁡                  (                      1            -                                          V                TH                                            V                CC                                              )                      +                            C          S                                      C            X                    +                      Δ            ⁢                                                  ⁢                          C              X                                          ×              ln        ⁡                  (                      1            -                                          V                TH                                            V                CC                                              )                    This gives:
      Δ    ⁢                  ⁢          C      X        =                    -        Th            ×              C        X        2                                      C          S                ×                  ln          ⁡                      (                          1              -                                                V                  TH                                                  V                  CC                                                      )                              -              Th        ×                  C          X                    
As the reference capacitance CS is, according to the prior art, appreciably greater than the capacitance CX of the electrode 4, the following equation is obtained for the variation ΔCX of the capacitance CX:
                              Δ          ⁢                                          ⁢                      C            X                          ≈                                            -              Th                        ×                          C              X              2                                                          C              S                        ×                          ln              ⁡                              (                                  1                  -                                                            V                      TH                                                              V                      CC                                                                      )                                                                        (        3        )            This equation (3) is known to the person skilled in the art.
Consequently, the variation ΔCX of the capacitance CX measurable by the capacitive sensor 3, (stated otherwise, the sensitivity of the latter), defined by equation (3) depends on numerous parameters: the value of the storage capacitance CS, the supply voltage VCC, the voltage threshold for stopping measurement VTH and especially chiefly on the capacitance of the electrode squared CX2. Now, the capacitance CX of the electrode 4 is difficult to control and varies as a function of the environment (temperature, humidity, etc.), thus degrading the value of the variation ΔCX of the capacitance CX and therefore the sensitivity and the performance of the capacitive sensor 3.
Moreover, the number of discharges x which conditions the duration of measurement, is proportional to the reference capacitance CS (cf. equation (2)), which is itself dependent on the other parameters and in particular on the desired variation ΔCX (cf. equation (3)). Thus for a variation ΔCX of the capacitance CX which is given, there corresponds a value of the reference capacitance CS and therefore a fixed number of discharges x (Th, VCC, VTH, and CX being fixed parameters). Consequently, the number of discharges x, that is to say the duration of charge transfer, or the duration of measurement of the variation ΔCX of the capacitance CX until detection, is fixed and cannot be optimized. Indeed, if the number of discharges x is reduced by two, for example, to reduce the duration of measurement, the reference capacitance CS is divided by two according to equation (2), and consequently, the variation ΔCX of the capacitance CX is degraded, since it is multiplied by two according to equation (3). With such a device, there is therefore no means of optimizing the duration of measurement of the capacitive sensor 3 without impacting the variation ΔCX of the capacitance CX, that is to say the sensitivity of the capacitive sensor 3.
However, the duration of measurement of the capacitive sensor must be extremely fast, since:                the door opening mechanism must be completely transparent to the driver. Indeed, the latter expects the opening of the door to be as fast as in the case of opening a mechanical handle, not equipped with a capacitive sensor 3,        the consumption of the capacitive sensor 3 must be minimized, since it operates for long periods when the vehicle is stopped. Now, consumption being related to the duration of measurement, if the duration of measurement is reduced, consumption decreases.        
However, as detailed hereinabove, given that the reduction in the duration of measurement brings about a degradation in the sensitivity of the capacitive sensor 3, this can cause overly late detections. Indeed, a degradation in the sensitivity of the sensor signifies that detection was carried out only when a large variation ΔCX of the capacitance CX was measured. A necessary compromise therefore exists between the duration of measurement and the sensitivity desired, that is to say the variation ΔCX of the capacitance CX desired. It will have been understood that there is a significant advantage in producing a capacitive sensor 3 for which the variation ΔCX of the capacitance CX is independent of the duration of measurement.
A device for measuring a variation ΔCX of the capacitance CX making it possible to alleviate these drawbacks is known from the prior art (cf. FIG. 3). In this instance, document FR 2 938 344 A1 describes a device for measuring a variation ΔCX of the capacitance CX furthermore comprising:                a third capacitance, called the measurement capacitance CM, linked to ground,        means (a switch S3) for charging this measurement capacitance on the basis of the supply voltage VCC, and        means (a switch S4) for discharging the measurement capacitance CM to the reference capacitance CS in a variable number of discharges n.        
This measurement capacitance CM makes it possible to carry out the measurement of the variation ΔCX of the capacitance CX in such a way that this variation is independent of the capacitance CX of the electrode 4 measured. This allows the optimization of the duration of measurement until the detection (that is to say the optimization of the number of charges and/or discharges) of the capacitive sensor 3 without impacting its variation ΔCX.
According to the invention described in document FR 2 938 344 A1, the charge transfer breaks down into two phases: acquisition and measurement.
The acquisition phase consists of a conventional transfer of charge from the capacitance CX of the electrode 4 into the reference capacitance CS. The difference with conventional charge transfer, described above, is that the charge transfer stops after a fixed number of discharges x and not when the voltage VS across the terminals of the reference capacitance CS attains a voltage threshold VTH.
The measurement phase consists of a transfer of charge, of a variable number of discharges n, of the measurement capacitance CM into the reference capacitance CS until the voltage VS across the terminals of the reference capacitance CS reaches the threshold voltage VTH.
During the acquisition phase, the charge of the capacitance CX of the electrode 4 is transferred into the measurement capacitance CS in the following manner:                1st step: the first step consists in charging the capacitance CX of the electrode 4 on the basis of the supply voltage VCC. Accordingly the first switch S1 is closed and the second switch S2 is opened,        2nd step: once the charging of the capacitance CX of the electrode 4 has terminated, the first switch S1 is opened,        3rd step: the discharging of the capacitance CX of the electrode 4 into the reference capacitance CS can begin. Accordingly, the first switch S1 remains open and the second switch S2 is closed,        4th step: once the discharging of the capacitance CX of the electrode 4 into the reference capacitance CS has been carried out, the second switch S2 is opened.        
The third and the fourth switch S3 and S4 are open during this acquisition phase. Consequently, the measurement capacitance CM is neither charged, nor discharged during this acquisition phase.
This charging and discharging cycle is repeated a predetermined and fixed number of times x.
During the measurement phase, the charge of the measurement capacitance CM is transferred into the reference capacitance CS until the voltage VS across the terminals of this capacitance attains a threshold VTH.                1st step: the first step consists in charging the measurement capacitance CM. Accordingly the third switch S3 is closed and the fourth switch S4 is opened,        2nd step: once the charging of the measurement capacitance CM has terminated, the third switch S3 is opened,        3rd step: the discharging of the measurement capacitance CM into the reference capacitance CS can begin. Accordingly, the third switch S3 remains open and the fourth switch S4 is closed,        4th step: once the discharging of the measurement capacitance CM into the reference capacitance CS has been carried out, the fourth switch S4 is opened.        
The first and the second switch S1 and S2 are open during this measurement phase. Consequently the capacitance CX of the electrode 4 is neither charged, nor discharged during this measurement phase.
This cycle is repeated until the voltage VS across the terminals of the reference capacitance CS attains the threshold voltage VTH. The variable number of discharges (called n) required to attain the threshold represents an image of the capacitance CX. The reference capacitance CS is thereafter completely discharged by closing the switch S in preparation for the next measurement.
Thus, according to document FR 2 938 344 A1, the variation ΔCX of the capacitance CX is no longer dependent on the capacitance CX of the electrode 4, but it is defined according to equation (4):
                              Δ          ⁢                                          ⁢                      C            X                          =                              Th            ×                          C              M                                x                                    (        4        )            
That is to say the variation ΔCX of the capacitance CX depends on the measurement capacitance CM, on the fixed number of discharges x of the electrode CX into the measurement capacitance CS and on the detection threshold Th. And n, the variable number of discharges of the measurement capacitance CM to the reference capacitance CS is defined by:
                    n        =                  -                                                                      C                  S                                ×                                  ln                  ⁡                                      (                                          1                      -                                                                        V                          TH                                                                          V                          CC                                                                                      )                                                              +                              x                ×                                  C                  X                                                                    C              M                                                          (        5        )            
The measurement capacitance CM being fixed, so is the fixed number of discharges x, and the detection threshold Th also being determined and fixed (since it is equivalent to a number of discharges n of the measurement capacitance CM into the reference capacitance CS corresponding to the threshold for detecting the driver's hand on the handle 6 of the door), it is therefore possible to choose the variation ΔCX of the capacitance CX by choosing in correspondence the values of CM, x, and of Th, independently of the value of the capacitance CX. Thus the variation ΔCX of the capacitance CX no longer depends on the value of this capacitance CX.
However, a major drawback of this device is the presence of parasitic residual capacitances originating from the switches S3 and S4 used to charge and discharge the measurement capacitance CM. The consequence of these residual capacitances is to limit the minimum value of the measurement capacitance CM, below which the variation ΔCX of the capacitance CX can no longer be improved (decreased). Generally, a switch exhibits a residual capacitance of 5 pF. The two switches S3 and S4 therefore exhibit an aggregate residual capacitance of 2×5=10 pF. The value of the measurement capacitance CM must be chosen as a function of this cumulated residual capacitance, and generally its value is chosen to be equal to this residual capacitance, i.e. of the order of 10 pF. The variation ΔCX of the capacitance CX (the smallest value of the variation ΔCX measurable) therefore attains a minimum when the measurement capacitance CM=10 pF and it can no longer be optimized with the prior art device described in document FR 2 938 344 A1. In an exemplary use, if Th=5, if x=170 and if CM=10 pF (minimum value due to the residual capacitances of the two switches S3 and S4), then the variation ΔCX of the capacitance CX is equal to 0.3 pF. Now, if it were possible to lower the value of the measurement capacitance CM, the variation ΔCX of the capacitance CX would be improved (decreased) proportionately but this is not possible since the circuit already exhibits a residual capacitance of 10 pF.